1. Field of the Invention
The present invention relates to a composite substrate, an elastic wave device, and a method for producing an elastic wave device.
2. Description of the Related Art
In recent years, for the purpose of improving the temperature characteristics of elastic wave devices, a composite substrate formed by bonding a thin piezoelectric substrate onto a support substrate having a low thermal expansion coefficient is used. For example, Patent Literature 1 discloses a composite substrate having a structure formed by bonding together a LT substrate (LT is the abbreviation of lithium tantalate) serving as a piezoelectric substrate and a silicon substrate serving as a support substrate via an adhesive layer formed of an epoxy adhesive.
Production of elastic wave devices with such a composite substrate has the following problems. First, since the composite substrate warps considerably in response to changes in temperature, during production processes for elastic wave devices at various process temperatures, the patterning accuracy is degraded or it becomes difficult to automatically handle the composite substrate. Second, the composite substrate has a low strength and the substrate becomes cracked during a heating process in production of elastic wave devices.
In order to overcome these problems, increasing the thickness of the support substrate in the composite substrate in Patent Literature 1 may be considered, for example. By simply increasing the thickness of the support substrate, the rigidity is enhanced and warpage of the composite substrate is suppressed. In addition, damage due to warpage of the substrate is also reduced. It has also been proposed that, in Patent Literature 2, a compensation layer that has substantially the same thermal expansion coefficient as a piezoelectric substrate and that has a thickness similar to that of the piezoelectric substrate is formed on a surface of the support substrate, the surface being opposite to another surface of the support substrate bonded to the piezoelectric substrate. In such a composite substrate, the piezoelectric substrate and the compensation layer similarly expand and contract in response to changes in temperature and hence warpage of the composite substrate is suppressed.